Thunderbird add-ons that I've either used for years, or tested and highly suggest.
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
Displays the address books in a sidebar in Thunderbirds 3-pane-window and can be toggled with the F4 key or a toolbar button.
Visit my Contacts Sidebar site for recent versions and news.
Add symbol in header plane to switch between compact and expanded view. Options to switch between one or two lines compact view, which buttons should be displayed in header pane. For Thunderbird 36 and older use version 2.0.9 (see version history)
Reinstates the extensions.checkCompatibility preference without respect to the current application version.
The "Swiss army knife" of folders. Turn your favorites into tabs - open folders and subfolders, move mails without scrolling around or searching. powerful + easy.
See new mail at a glance after automatic filtering - supports quickFilters addon!
Extracts all attachments from selected messages and then can delete, detach or mark-read.
Add more items to Add-ons Manager context menu.
Synchronizes selected addressbooks with other instances of thunderbird
using ftp/webdav/imap servers
Allows for customization of the folder pane. Accounts can be rearranged and the startup folder can be chosen.
Check the message contents and addresses before you send the message...
Displays icon for user agent of received mails.
Thunderbird Message Filter Import/Export is a thunderbird/mozilla extension to import/export message filters.
This addon adds month tabs into Lightning calendar month view